1. Technical Field
The present disclosure relates to a line driver circuit and more particularly, to a line driver circuit having means for stabilizing a signal that is output through a transmission line.
2. Discussion of the Related Art
In general, in semiconductor devices having a plurality of circuits, signals are transmitted/received through transmission lines connecting the circuits, and power dissipation increases due to capacitances of the transmission lines. A line driver circuit for lowering power dissipation by reducing a swing width of a signal that is transmitted/received through a transmission line is disclosed in U.S. Pat. No. 5,023,472. The line driver circuit will now be described with reference to FIG. 1.
FIG. 1 is a circuit diagram of a conventional line driver circuit. In this line driver circuit, a swing width of a signal transmitted/received through a transmission line can be reduced using a charge sharing method.
The line driver circuit of FIG. 1 includes a pull-up circuit and a pull-down circuit that are connected in series between a first voltage VDD and a second voltage VSS. The pull-up circuit includes a first switch SW1 and a second switch SW2 that are connected in series to the first voltage VDD. Further, the pull-up circuit includes a first capacitor C1 connected to a common node of the first switch SW1 and the second switch SW2.
The pull-down circuit includes a third switch SW3 and a fourth switch SW4 that are connected in series to the second voltage VSS. A second capacitor C2 is connected to a common node of the third switch SW3 and the fourth switch SW4. A resistance component and a capacitance component of the transmission line are referred to as a resister RL and a capacitor CL, respectively.
When the first switch SW1 is turned off, and the second switch SW2 is turned on, charge sharing occurs between the first capacitor C1 and the capacitor CL of the transmission line, with the first capacitor C1 being pre-charged to the first voltage VDD and the capacitor CL being pre-charged to a low level. As a result, a signal OUT transmitted through the transmission line becomes a high level.
Thereafter, when the third switch SW3 is turned on, and the fourth switch SW4 is turned off, charge sharing occurs between the second capacitor C2 and the capacitor CL of the transmission line, with the second capacitor C2 being pre-charged to the second voltage VSS and the capacitor CL being pre-charged to the high level. As a result, the signal OUT transmitted through the transmission line becomes a low level.
This can be expressed by the following equation.VH=VDD*C1*(C1+C2)/[CL*(C1+C2)+C1*C2]VL=VDD*C1*CL/[CL*(C1+C2)+C1*C2]  [Equation 1]
(VH: high level voltage, VL: low level voltage)
Assume that C1=C2=CL in the equation 1, then a swing width VH−VL of the signal OUT is ⅓*VDD.
In other words, when the signal OUT is toggled, the required charge quantity can be reduced by ⅓, thereby lowering power dissipation by ⅓.
When the line drive circuit is powered up, or when the circuit has not operated for a long time, a node of the transmission line is required to be initialized so as to make the signal OUT become the high level or the low level. Otherwise, an abnormal voltage may be generated while the line drive circuit operates normally, thereby causing errors.
Also, when charge sharing occurs between the capacitor CL of the transmission line and the capacitors C1 and C2 of the pull-up circuit and the pull-down circuit, a leakage current may be generated. Thus, the signal OUT may not reach to the high level or the low level. While the leakage current increases, the signal OUT may be shifted to the opposite level.
The conventional line driver circuit does not have a voltage initializing means or a means for compensating for changes in signal level caused by the leakage current, which may make the signal OUT unstable.